The usual commercial method for preparing commercial quantities of phosphorus sulfides, particularly, phosphorus pentasulfide is to continuously introduce liquid phosphorus and liquid sulfur into a boiling mass consisting of phosphorus pentasulfide to effect a continuous reaction between the phosphorus and sulfur. The product may be removed from the reactor by continuous distillation of the reaction product. The control of the reaction is achieved by the rate of addition of the reactants and the proportions being added. Such a process is described in U.S. Pat. No. 2,794,705 to Hudson. While total reactant feed rate is mostly controlled by observing the temperature of the reaction, the control of proper proportions of each reactant being added can only be definitively provided by analysis of the product. Usually several hours are required to determine the result of readjusting feed rates, temperature, sojourn time and cooling rates based upon analytical work.
Because the reaction of phosphorus and sulfur is rapid and exothermic at the elevated temperatures commonly employed, many attempts have been made to render the reaction more controllable and efficient. In S.U. 1,301,775 the heat evolution from the reaction of phosphorus and sulfur is controlled by conducting the reaction at a temperature of 130.degree. C. to 175.degree. C. and providing a sulfur to phosphorus ratio of 3.5-4.0:1 until the temperature ceases to increase and then adding additional phosphorus to provide the desired stoichiometric amount, i.e., sulfur to phosphorus ratio of 5:2.
In another attempt to control the process to provide efficiency, there has been developed a continuous loop reactor containing a liquid reaction mixture which is continuously circulated. The liquid phosphorus and sulfur reactants are introduced into the loop so as to avoid a large concentration of phosphorus by feeding the phosphorus into the loop at a point in which there is a concentration of sulfur. The phosphorus sulfide product is removed from the reactor in the gaseous state. Such a process is described in U.S. Pat. No. 5,198,202 to Courant et al.
Due to the kinetics of the reaction between phosphorus and sulfur at such high temperatures as is normally encountered when removing product by distillation there is a vibration problem. One solution to this problem is to either use smaller reactors or to use a two stage reactor. In a first stage, usually a small reactor, phosphorus and sulfur are fed at a ratio to provide a phosphorus content of about 28%. In the second stage, the desired amount of phosphorus is added to provide the desired sulfide. While the in the first stage the reactor is operated at boiling point of the mixture, the second stage is operated at a temperature as low as 300.degree.-353.degree. C. Such a process is described in East German patent 119,198 to Strauss.
While the above noted prior art is directed to improved safety of the process further advances are needed to provide a truly efficient process which is also more safely and accurately operated.